Viruses which pollute water are excreted with faeces or urine from human beings and many species of animals. These viruses include polio virus, coxsackie virus, echo virus and other entero viruses, adeno virus, reo virus, rota virus, hepatitis A virus, etc. which can cause acute infectious non-bacterial gastroenteritis. These viruses are, mostly, present in relatively large numbers in sewage contaminated water or ground water. As per World Health Organization (WHO) recommendations drinking water should be free from any viruses and no virus should be detectable in samples of 100-1000 liters of directly reclaimed drinking water as described in a report of a WHO Scientific Group published in WHO Technical Report Series 639, WHO, Geneva (1979). For public health, the development of reliable, sensitive and practical methods for detecting viruses in large quantities of water is very essential. Most of the methods of virus concentration have been developed for polio virus. Viruses can be concentrated from water samples by viradel (virus adsorption-elution) technique. The virus gets adsorbed presumably by both electrostatic and hydrophobic interactions between the virus and the filter matrix.
However, these previously developed methods for concentration of entero viruses from water have proved to be of limited value when applied to the concentration of rota viruses. Although, ultrafilteration is an extremely useful method for concentrating viruses, it is only effective when applied to relatively small volumes of water, which have low turbidities. Since, ultrafilteration depends on the physical size of the virus particle the pore size has to be in nanometer range. This type of filter will rapidly clog, thus limiting the water volume which can be processed. In addition, the bulky equipment used for concentration of viruses limits sampling to readily accessible areas as described by G. A. Toranzos and C. P. Gerba, in J. Virological Methods, 24, 131 (1989).
Concentration of viruses from water has been performed by using both electronegative as well as electropositive filters as given by G. Sansebastiano et al. in L'Igiene Moderna, 93, 785 (1990). The conventional micro porous electronegative (having negative charge on the surface) adsorbent filters adsorb viruses more efficiently in the presence of multivalent cations such as Al.sup.3+ and Mg.sup.2+ on/or at low pH usually 3.5. The efficient virus adsorption occurs only if water is acidified to pH 3.5 and/or multi cation salts are added. The major drawback is the extensive modification of the water sample. The strongly acidic and basic pH levels are utilized for the formation of precipitates during the reconcentration and their susceptibility to the variation in the quality of water sample.
The electropositive filters are composed of fibre glass or cellulose acetate and positively charged organic polymer resin as described in `Standard Methods for the Examination of Water and Waste-Water` edited by Andrew D. Etaon et al., 19th edition, American Public Health Association, Washington D.C., 1995. The microporous filters which are positively charged are advantageous over negatively charged filters as the virus adsorbence occurs for most natural and tap water in the pH range.about.pH 5-9, because they reduce or eliminate the need for either acid or salt addition to obtain virus adsorption as described by M. D. Sobsey and B. L. Jones, in Applied & Environmental Microbiology, 37, 588 (1979).
Bocchi et. al (J. Mater. Sci., 26, 3354 (1991)) reported that the polypyrrole deposited on glass fibre membrane is very efficient in capturing viruses as compared to the same polpyrrole deposited on paper. But by repeating the same experiment the inventors of the present invention obtained conductivity.about.10.sup.-2 .OMEGA..sup.-1 cm.sup.-1 contrary to their reported Value of 10 .OMEGA..sup.-1 cm.sup.-1 using glass fibre substrate. Further, the electrical conductivity reported by Bocchi et. al (1991) in the same publication was 50 .OMEGA..sup.-1 cm.sup.-1 using filter paper where as the present inventors obtained the conductivity.about.1 .OMEGA..sup.-1 cm.sup.-1 for the polypyrrole impregnated filter paper. Moreover, the adhesivity of the polypyrrole on the glass fibre was poor as a result of which the deposition was flaky and could easily come out when water is poured. In addition, the membrane was washed with dilute hydrochloric acid both in case of fibre glass and paper membrane questioning thereby the potability of the filtered water.
The conventional micro porous filters suffer from three main limitations: (a) simple suspended matter tends to clog the adsorbent filter thereby limiting the volume that can be processed and possibly interfering with elution process; (b) dissolved and colloidal organic matters in some waters can interfere with virus adsorption to filters presumably by competing with viruses for adsorption sites and they also can interfere with virus elution; (c) viruses adsorbed to suspended matter may be removed in any cleaning process applied before virus adsorption as described in `Standard Methods for the Examination of Water and Waste-Water` edited by Andrew D. Eaton et al., 19th edition, American Public Health Association, Washington D.C., 1995. Thus, the majority of processes used to treat sources of potable water are capable of reducing virus numbers but with possible exception of high-grade disinfection, none of them can be relied upon to remove the virus under all circumstances.
In the conventional water treatment the rapid sand filtration or micro straining results in negligible reduction in virus number. Flocculation processes, generally combined with rapid filtration, have been explained to remove 60-99% viruses. However, viruses removed by flocculation is not inactivated. Lime flocculation, which is often applied to renovated waters, is very effective provided the alkaline conditions (pH&gt;11.5) are maintained for at least 1 hr. Adsorption by activated carbon can remove viruses but the adsorbed viruses may be liberated at later stage when organic material competes for adsorption sites. Disinfecting process can destroy viruses when used correctly. The most widely used disinfectants are chlorine and ozone. The chlorine is highly effective virucide whereas combined chlorine is far low effective. The amount of chlorine required will depend upon quality of water, in particular its pH value, ammonia and organic solvent. In circumstances where drinking water is likely to become contaminated, it is not possible to achieve complete protection. Moreover, when water containing organic matter is chlorinated, carcinogenic compounds like trihalomethanes may be formed. Ozone has also been shown to be effective viral disinfectant preferably for clean water, but it is not possible to maintain a residual in distribution system. Under the average conditions of operations of many treatment plants, it can be expected that viruses from contaminated water sources may penetrate the drinking water distribution systems.
All sewage treatment processes remove or destroy viruses to some degree. Primary sedimentation can remove a significant proportion of viruses (up to 50%) owing to their association with solid matter. Of secondary treatment procedures, the activated sludge process removes 60-99% of viruses present. Chemical coagulation is regarded as one of the most effective single step treatments. Lime is probably most efficient, since it not only removes viruses physically but also inactivates them by exposing them to high pH. The filtration of coagulated effluents is an additional important process. Adsorption methods using clays, coal or activated carbon can remove viruses to some extent, but the process is not efficient. The application of wastewater to land can be a valuable tertiary treatment and is being used successfully in number of countries. Little evidence is available about the survival of viruses in the soil or run off water but a number of studies show clearly that they can survive for long periods in soil and may be eluted by heavy rainfall. Under these circumstances, some form of disinfection must be applied to render waste water safe before discharge into environment. Chlorine is widely used for this purpose but its efficacy is reduced by the presence of organic material, inadequate contact time, insufficient dose, temperature, pH and presence of ammonia. Due to these factors, chlorination becomes ineffective. So the conventional treatment plants are not 100% efficient in removing virus.
Assuming that a population of one million consumes conventionally treated water having 1 infectious unit per 20 liters and assuming each person drinks 1 liter of this water daily, then 50,000 persons ingest one infectious particle per day. Due to immunity of those persons if only 1% gets infection i.e. 500 persons per day or 1,82,500 persons per year they can still further act as carrier to infect their contacts. If 10 in 500 infected persons become ill, 3650 persons get clinical disease every year. On the basis of these considerations WHO concluded that the presence of even a few enteric viruses in a large volume of drinking water should be prevented as described in a report of a WHO Scientific Group published in WHO Technical Report Series 639, WHO, Geneva (1979).